In an attempt to control a hydraulic elevator with precision approximating the more sophisticated and usually more expensive traction elevators, feedback control is used. But, even using feedback control, comparable performance has been difficult to achieve. The main problem is the dynamic characteristics of the fluid. The fluid viscosity shifts with ambient temperature and also from the heating that occurs as the elevator car is raised and lowered. These variables produce some measure of unpredictability in the motion of the elevator car. Different levels of feedback have been utilized, but typically these approaches are expensive and lower system efficiency because they require excess pump capacity.
A technique illustrating feedback is shown in U.S. Pat. No. 4,205,592, where the flow through the valve and to an object, such as a hydraulic elevator, is passed through a flow meter that includes a potentiometer. As the flow increases, the output voltage associated with the motion of the potentiometer wiper changes, manifesting the magnitude of the flow. U.S. Pat. No. 4,381,699 shows a similar type of valve control.
U.S. Pat. No. 4,418,794 is illustrative of the type of valve that may be used in systems that do not sense the fluid flow but, using a larger feedback loop, perhaps sense the position of the elevator car and control the operation of the valve.